1. Field of the Invention
The present invention relates to the field of temperature measurement technology. It concerns a high temperature probe, in particular for use in the hot gas stream of a gas turbine at temperatures of up to 1200.degree. C., comprising a metallic probe tube having an internal drilled hole running in the direction of the tube axis of the probe tube and a measurement space arranged at the lower end, which space is separated from the internal drilled hole by a constriction and is in contact with the surroundings of the probe tube, and also at least one thermocouple which is conducted downwards in an insulated manner in the internal drilled hole and through the constriction into the measurement space.
Such a high temperature probe is known, for example, from the publication DE-A-2 413 909.
2. Discussion of Background
In the operation of gas turbines it is usually necessary to measure the temperature of the hot combustion gases, which can be from a few 100.degree. C. to above 1200.degree. C. For this purpose, there have already been proposed high temperature probes which can be inserted into appropriate openings in the hot gas housing of the turbine and project into the hot gas stream by means of a probe tube which contains the actual measuring element at the lower end.
Thus, the publication U.S. Pat. No. 5,180,227 discloses a high temperature probe which operates optically and in which an optical sensor element of sapphire is arranged in a measurement space formed at the lower end of the probe tube. The measurement space is connected with the surrounding hot gas space via an inlet opening fitted below the sensor element and an outlet opening fitted at the level of the sensor element. The gas entering from the hot gas stream through the inlet opening is, owing to the arrangement, braked in the measurement space and flows around the sensor element at a very low velocity before again leaving through the outlet opening. The measurement space is therefore also known as a "stagnation chamber". The probe can be used up to temperatures of 1300.degree. C. and is cooled in the upper part of the probe tube by means of air or another gas which is fed in at the probe flange and is allowed to leave again just above a thermal barrier which separates the sensor element from the upper part of the probe.
Furthermore, the publication mentioned in the introduction discloses a high temperature probe for use in gas turbine aircraft engines at temperatures up to 800.degree. C., which probe operates using a thermocouple as temperature sensor. The thermocouple is conducted in a metal-sheathed cable having mineral insulation downwards into a probe tube and through a sealing constriction into a measurement space so that the connection point between the two wires of the thermocouple is arranged approximately in the middle of the measurement space. Here too, contact with the hot gases to be measured is achieved by provision of an inlet opening fitted below the connection point and an outlet opening fitted above the connection point, which openings connect the measurement space with the surrounding hot gas space. Here too, the hot gas is held up and calmed in the measurement space by the arrangement and configuration of the openings.
In the known probes, the hot gas to be measured is strongly braked in respect of its flow velocity and is brought into contact with the respective sensor element in an almost static state. This reduces the heat transfer between gas and sensor element, which shows up in an increased time constant and a greater deviation between actual and measured temperature.